1. Field of the Invention
The present invention relates to a polymer solar cell and a method of manufacturing the same, and more specifically, to a polymer solar cell comprising an electron-accepting layer between a photoactive layer and a second electrode, capable of realizing increased power conversion efficiency. In particular, high power conversion efficiency can be attained even in a low-temperature thermal annealing process.
2. Description of the Related Art
In general, a solar cell is a photovoltaic device used for the conversion of solar light into electrical energy. A solar cell is usable without limitation, is environmentally friendly, unlike other energy sources, and, is thus expected to become an increasingly important energy source over time.
Conventionally, a silicon solar cell made of monocrystalline or polycrystalline silicon has been mainly utilized. However, the silicon solar cell suffers from disadvantages because it has a high manufacturing cost and cannot be applied to a flexible substrate. As an alternative to the silicon solar cell, thorough research into polymer solar cells is currently ongoing.
The polymer solar cell may be manufactured through spin coating, ink-jet printing, roll coating, or doctor blading, and therefore the manufacturing process is simple resulting in a low manufacturing cost. Further, the use of a polymer solar cell is advantageous because a large area may be coated, a thin film may be formed even at low temperatures and, almost any kind of substrate, including a glass substrate and a plastic substrate, may be used.
In addition, solar cells having various shapes may be manufactured, such as curved or spherical plastic molded products, which may also be bent or folded so that they are easily portable. When making use of the above advantages, a solar cell may be manufactured that can easily be attached to people's clothes, bags, or be mounted to portable electrical or electronic products. In addition, when a polymer blend film, having high transparency to light, is attached to the glass windows of buildings or the glass windows of automobiles, it can generate power while simultaneously allowing a person to see through the window. Consequently, polymer solar cells have a broader range of application than opaque silicon solar cells.
Although the polymer solar cell possesses the above advantages, it is unsuitable for practical use because the power conversion efficiency thereof is low and the lifetime thereof is short. That is, by the end of the 1990s the power conversion efficiency of the polymer solar cell was only about 1%. However, since the year 2000, the performance of the cell has begun to greatly increase through improvements in the structural morphology of the polymer blend. Presently, in the case where the power conversion efficiency of the polymer solar cell is measured under solar light conditions of AM 1.5 global 100 mW/cm2, a unit device having a small area (0.1 cm2 or less) has power conversion efficiency of about 4 to about 5%, and a device having an area of 1 cm2 has power conversion efficiency of about 3% ((M. A. Green, K. Emery, D. L. King, Y. Hishikawa and W. Warta, Prog. Photovolt. Res. Appl. 14, 455-461(2006)).
Typically, a polymer solar cell comprises a first electrode, a second electrode, and a thin film layer composed of a conjugated polymer or a conductive polymer having semiconductor properties and, an electron acceptor between the first electrode and the second electrode.
An example of a polymer useful in a polymer solar cell, is a conductive polymer such as polythiophene or p-phenylene vinylene (“PPV”) derivatives, that function as an electron donor. When the conductive polymer absorbs light having a wavelength not less than an energy band gap, it is excited to an exciton. The exciton binding energy of the conductive polymer typically ranges from 0.1 to 1.0 eV, which is considerably greater than thermal energy (about 0.025 eV) at room temperature. Accordingly, since the conductive polymer has a low probability of being separated into free electrons and complementary positively charged holes, solar cells using a thin film composed exclusively of the conductive polymer have very low power conversion efficiencies of about 0.1% or less.
In order to increase the free electron production efficiency of a single film comprising the conductive polymer, the use of a double film composed of a conductive polymer and an electron acceptor has been proposed. However, the exciton diffusion length in the polymer semiconductor, is of about 3 to about 10 nanometers (nm). Thus, in the thin double layer comprised of the conductive polymer and an electron acceptor, free charges are produced only in the narrow region corresponding to the heterojunction interface, which is about 3 to about 10 nm thick, in the total thin film layer which is about 100 nm thick. Consequently, the charge production efficiency is still low.
With the goal of increasing the heterojunction interface between the conductive polymer and the electron acceptor, research is being conducted into polymer solar cells that include a blend layer of conductive polymer and an electron acceptor. As such, the heterojunction interface between the conductive polymer and the electron acceptor is distributed over the entire internal portion of the thin film, and thus the production of free charges may be effectively realized throughout the entire thin film layer. For example, power conversion efficiency of about 3.5% has been reported using a blend film of “P3HT” (poly(3-hexylthiophene)) and “PCBM” ([6,6]-phenyl-C61 butyric acid methyl ester) and a thin LiF buffer layer at the junction interface with an Al electrode. [F. Padinger, R. S. Rittberger, N. S. Sariciftci, Adv. Func. Mater., 13, 85(2003)] However, such a polymer solar cell still has lower power conversion efficiency than other thin-film solar cells, and extensive effort is required to further increase the power conversion efficiency of polymer solar cells.
In the polymer solar cell, various attempts have been made to increase the crystallinity of the polymer through thermal annealing at about 120 to about 160° C., in order to improve the low charge mobility of the polymer due to the disordered structure of the polymer nanocomposite.
In the case where the solar cell is manufactured on a plastic substrate, it is difficult to perform thermal annealing due to problems related to the thermal deformation of the plastic. Therefore, decreasing the thermal annealing temperature of the polymer blend is considered to be an important step in the development of plastic solar cells.